Tetrafluorophthalic acids and derivatives



United States Patent 3,240,792 TETRAFLUOROPHTHALIC ACIDS AND DERIVATIVES Colin Russell Patrick and John Colin Tatlow, Birmingham, England, assignors to Imperial Smelting Corporation (N.S.C.) Limited, London, England, a British comp y N0 Drawing. Filed Feb. 25, 1965, Ser. No. 435,376 Claims. (Cl. 260346.3)

This application is a continuation-in-part of an earlier application Serial No. 181,471, filed on March 21, 1962, now abandoned.

This invention relates to fluorinated aromatic dicarboxylic acids and derivatives thereof and to a process for the preparation of these compounds.

Highly fluorinated aromatic compounds are valuable as synthetic intermediates for the production of polymers, pharmaceuticals, dyestuffs, heat-exchange fluids, dielectric liquids and specialised solvents. The compounds of the invention are key intermediates for some of these purposes, as will be set forth in more detail below.

The invention has as its main object the provision of three novel compounds:

(1) Tetrafluorophthalic acid, o-C F (COOH) (2) Tetrafluoro-isophthalic acid, m-C F (COOH) (3) Tetrafluoro-terephthalic acid, p-C F (COOH) Examples will be given below to show how these acids can be used to prepare:

Dirnethyl tetrafluoroterephthalate p-C F (COOCH Tetrafluorophthalic anhydride 3,4,5,6-tetrafluorophthalimide 1,2,3,4-tetrafluorobenzene, and 1,2,4,5-tetrafluorobenzene A further object of the invention is to provide a method of preparing a tetrafluorophenyl dicarboxylic acid, which method includes the step of subjecting a perfluoroxylene to hydrolysis with a strong acid. Conveniently, this hydrolysis can be carried out using fuming sulfuric acid. Temperatures in the range 100-200 C. are preferred for carrying out the hydrolysis and a temperature around 150 C. can conveniently be used.

The perfluoroxylenes, which are used as the starting materials in the process according to the invention, are best prepared by passing a perfluoro(dimethylcyclohexane) over a heated metal such as iron or nickel at a temperature in the range 300 C. to 600 C. as described in U.S.P. 3,000,976. Defluorination of perfluoro(1,2-dimethylcyclohexane) yields decafluoro-o-xylene and defluorination of the corresponding 1,3- and 1,4-compounds yields the metaand para-xylenes respectively.

i 3,240,792 Patented Mar. 15, 1966 A general object of this invention is the preparation of fiuorinatedcarboxylic acids and their derivates from readily-available organic starting materials and a process is envisaged comprising the steps of: (i)35l( )luorinating a xylene using cobalt trifluoride at 300- (i-i) Separating the perfluoro(dimethyl cyclohexane) formed,

(iii) Passing the said perfluoro(dimethylcyclohexane) over heated iron at 450500 C. to form a perfluoroxylene, and

(iv) Subjecting the said perfluoroxylene to hydrolysis with a strong acid.

One of the valuable characteristics of the new acids is the fact that they can be readily decarboxycated to give valuable fluorocarbon products whose uses are described below. Thus 1,2,3,4- and l,2,4,5-tetrafluorobenzene are prepared by heating tetrafluorophthalic acid and tetrafluoroterephthalic acid respectively with soda lime and collecting the tetrafluorobenzene produced.

The present invention provides new tetrafluoroaromatic compounds, e.g., acids, anhydrides, fluorocarbons, etc. Although there is a structural similarity between these and the corresponding unfluorinated compounds, it could not be foreseen that the methods used would give the desired compounds in high yield and certain chemical evidence pointed in the other direction. Thus the prior art discloses that the preparation of C F COOH from C F .CF by hydrolysis with concentrated sulphuric acid proceeds in low yield and at a slow rate (J. Amer. Chem. Soc. 73, 1366 (1951)). In contrast, the hydrolysis reaction of the invention proceeds rapidly and high yields of the tetrafluoroaromatic dicarboxylic acids are obtained.

As is well known, the reactions of highly-fluorinated aromatic compounds are unpredictable and these compounds show diiferences from the reactions of other aromatic compounds.

Examples of such anomalous reactions are:

(a) C F SO H is unstable and decomposes on storage. (J. Chem. Soc. (1959), 166) whereas C H SO H is a stable and widely-used chemical intermediate,

(b) Halogenation of C F H does not proceed under the mild conditions which are successfully used in the case of CGHB (c) The diazonium salt of pentafluoroaniline (Chem. and

Ind. (1958), 630) gives anomalous reactions,

((1) Carbonation of C F MgBr in ether proceeds in extremely poor yield (Nield, Stephens and Tatlow, J.C.S (1959), 166) and the same Grignard does not react with ethylene oxide in ether solution (Tetrahedron, 47, p. 3575),

(e) C F .CH cannot be oxidised to C F .COOH except in very low yield. (Birchall and Haszeldine J.C.S., 1961, p. 3719.)

The tetrafluor-ophenyl dicarboxylic acids are crystalline solids. They undergo chemical reactions such as esterification upon heating the acid with an alcohol, formation of acid chlorides when the acid is treated with phosphorous pentachloride, and tetrafluorophthalic acid forms an anhydride which reacts with ammonia to form tetrafluorophthali-mide. Thus the compounds of the invention are useful intermediates for the production of such compounds as esters which are heat-stable and radiationstable fluids. Further, polymers are prepared by reacting the acid chlorides with a diamine, e.g. tetrafluoro-pphenylene diamine, and these novel products are useful as heat-transfer media, dielectrics, etc. In addition, tetrafluoroterephthalic acid, when condensed with glycols, forms polymers which are new members of a known class, typified by e.g. Dacron, Terylene (registered trade? marks). However, by virtue of containing a fluorinated 3 aromatic nucleus these new polymers have different heatresistant characteristics from the known polymers, as does polytetrafluoroethylene when compared with ethyle-ne.

An important use for the unfluorinated anthranilic acid and for phthalic anhydride is in the preparation of dyestuffs (phenolphthalein, fluoresceins, rhodamine dyes, etc.). The compounds provided by the present invention can also be elaborated into dyestuffs of these classes which contain fluorinated aromatic rings and thus they constitute valuable intermediates. Such reactions are set out in detail in Schmidt, Organic Chemistry, 6th edition, pp. 488 et. seq.

The tetrafluorobenzenes are useful intermediates for the production of C F -compounds generally and in themselves constitute heat-transfer or hydraulic fluids or dielectric liquids.

The invention will be further described with reference to the following examples:

EXAMPLE 1 Preparation of perflaoro-O-xylene Perfluoro-1,2-dimethylcyclohexane (9 g.) was passed in a stream of nitrogen flowing at 2 l./hr. through the defluorination tube which was heated to 460 C. The product (6 g.) was separated by preparative-scale gas chromatography to give starting material (3.1 g.) and perfluoro-o-xylene (1.8 g.) B.P. 128, 11 1.3670 (found: C, 33.2; F, 66.2. C F requires C, 33.6; F, 66.4%). Mass spectrometry (principal mass peaks): 286, C F C F 217, C F Light absorption in ethanol (c., 0.085 g./l.); max. 2720 (e: 1753).

EXAMPLE 2 Preparation of perfluoro-P-xylene Perfluoro-1,4-dimethylcyclohexane (9.0 g.) was treated as described above (reactor temperature 460 C.). The product (6.0. g.), separated as before, gave starting material (3.5 g.) and perfluoro-p-xylene (1.65 g.), B.P. 122, n 1.3621 (found: C, 33.9; F, 66.3%). Light absorption in ethanol (c., 0.047 g./1.); max. 2850 (e=2590).

An object of the invention is to provide new acids from these xylenes.

EXAMPLE 3 Preparation of tetrafluorophthalic acid Perfiuoro-o-xylene (6.4 g.) and fuming sulphuric acid (20% sulphur trioxide; 4 ml.) were heated at 150 and shaken in a sealed tube for 12 hr. The tube was cooled in liquid air, then opened, and its contents were poured on ice. The resulting solution was made alkaline, extracted with ether and acidified. The acid solution was extracted continuously with ether for 12 hr. and the extract was decolorised (charcoal), dried (MgSO and evaporated to leave a white solid (5.8 g.), M.P. 151153, which was re-crystallised from o-xylene to give tetrafluorophthalic acid (5.0 g.), M.P. l53154 (found: C, 40.3; H, 1.0; 31.6%; equiv., 117. C H F O requires C, 40.3; H, 0.8; F, 31.9%; equiv., 119). The infra-red spectrum showed a strong band at 1735 CITL"1 (CO).

The acid (0.3 g.) gave in the usual way di-(S-benzylthiouronium) tetrafluorophthalate (0.2 g.), M.P. 205 (decomp.) (from water) (found: C, 50. 8; H, 4.0 F, 13.4. C H F N O S requires C, 50.5; H, 3.9; F, 13.3%). Treatment of the acid (0.2 g.) with an excess of aniline in ether gave anilinium hydrogen tetrafluorophthalate (0.2 g.), M.P. 204-205" (decomp.) from acetone-chloroform (found: C, 50.8; H, 2.9. C H F NO requires C, 50.8; H, 2.7% As an example of a further derivative, a solution of tetrafluorophthalic acid (5.5 g.) in water (20 ml.) was neutralised with aqueous sodium carbonate. A solution of silver nitrate (20 g.) in water (50 ml.) was added and the precipitated disilver salt (9.5 g.) filtered off (found: C, 21.4. C Ag F O requires C, 21.2%

4 EXAMPLE 4 T etrafluorophtlzalic anhydride Tetrafiuorophthalic acid (2.4 g.) was refluxed with trifluoroacetic anhydride (5 ml.) for 2 hr. Trifluoroacetic acid and trifiuoroacetic anhydride were removed by distillation under reduced pressure, and the residue was sublimed at /0.05 rn-m., to yield tetrafluorophthalic anhydride (2.0 g.), 9495.5 (found: C, 43.4%; equiv., 110. C F O requires C, 43.6%; equiv., The infra-red spectrum showed a band at 1805 cm.-

EXAMPLE 5 T etrafluorophthalimide The temperature of a mixture of tetrafiuorophthalic anhydride (2.5 g.) and aqueous ammonia (d. 0.88; 3 ml.) was gradually increased to 280 during 1 hr. The solid product (2.5 g.) sublimed at /0.05 mm., to give tetrafluorophthalimide (1.5 g.), M.P. 2l0-211 (found: C, 44.2; H, 0.8; F, 34.5. C HF NO requires C, 43.8; H, 0.5 F, 34.7%).

This imide (0.3 g.) was added to a solution of xanthhydrol (0.3 g.) in glacial acetic acid (3 ml.) and heated in a boiling water bath for 30 minutes. The precipitate was re-crystallised from dioxan-water (3.2) to give the xanthhydrol derivative (0.15 g.) M.P. 255- 256 (found: C, 63.3; H. 2.3. C H F O N requires C, 63.2; H, 2.3%).

EXAMPLE 6 Preparation of 3,4,5,6-tetrafluoroanthranilic acid To a cooled solution of sodium hydroxide (1.7 g.) and bromine (1.3 g.) in water (10 ml.) was added finely powdered tetrafluorophthalimide (1.7 g.). A solution of sodium hydroxide (1.1 g.) in Water (5 ml.) was added rapidly and the resulting solution heated to 80 for 1 minute and then cooled to 15. The solution was acidified with concentrated hydrochloric acid, and the resulting precipitate (1.0 g), M.P. -139", removed by filtration. Reprecipitation from alkaline solution and recrystallisation from water gave 3,4,5,6-tetrafluoroanthranilic acid (0.5 g.), M.P. 141142 (found: C, 40.5; H, 1.6. C7H3F4NO2 requires C, 40.2; H, 1.4%). The infra-red spectrum showed bands at 1675 (CO), 3400, 3620 (N-H), and 3000 cm. broad (O-H).

The acid (0.2 g.) gave in the usual way S-benzylthiouronium 3,4,5,6-tetrafluoroanthranilate (0.15 g.), M.P. 205 (found: C, 48.2; H, 3.7. C H F N O S requires C, 48.0; H, 3.5%).

EXAMPLE 7 T etrafluoroterephthalic acid Perfluoro-p-xylene (6.0 g.) was treated with concentrated sulphuric acid (6 ml.) as for its isomer: Recrystallisation from water of the solid obtained gave tetrafiuoroterephthalic acid (4.0 g.), M.P. 283-284 (found: C, 40.2; H, 0.7; F, 31.5%), max. 1725 cm.- (C=O).

The acid (0.2 g.) gave a di-(S-benzylthiouronium) salt (0.3 g.), M.P. 211-212 (found: C, 50.7; H, 4.1%).

With aniline in ether the acid (0.26 g.) afforded a dianilium salt (0.20 g) (from acetone-chloroform), M.P. 205 (decomp.) (found: C, 56.6; H, 3.6. C H F N O requires C, 56.6; H, 3.8%).

EXAMPLE 8 DimethyItetrafluorotereph thalate The acid (1.1 g.) and concentrated sulphuric acid (1.0 ml.) were refluxed for 5 hr. in methanol (10 ml.). When cooled the precipitate which formed was filtered off and recrystallised from methanol to give dimethyltetrafluoroterephthalate (0.7 g.), M.P. 79.80 (found: C, 45.4; H, 2.0. C H F O requires C, 45.1; H, 2.3%

EXAMPLE 9 Decarboxylations of the tetrafluorophthalic acids NoTE.The melting points of the other two isomers were as follows:

C. Tetrafluorophthalic acid (0.43 g.) and soda-lime (1.0 Tetrafluomphthalic F 153-154 g.) were mixed intimately and the temperature increased 5 Tetmfluoroterephthahc and 283 284 gradually to 300 C. The distillate (0.20 g.) was shown by infra-red spectroscopy to be 1,2,3,4-tetrafluorobenzene.

In an analogous fashion, tetrafluoroterephthalic acid (0.51 g.) afforded 1,2,4,5-tetrafiuorobenzene (0.22 g.) with a correct infra-red spectrum.

EXAMPLE 1O Hydrolysis of decafluoro-m-xylene to tetrafluoroisophthalic acid Decafluoro-m-xylene (50 g.) and oleum ml.) were shaken in a ml. flask at C. for 12 hours. On cooling, the product Was poured into ice. The aqueous solution obtained Was continuously extracted with ether for 12 hours. The ether was distilled 01f leaving crude te-trafiuoro-iso-phthalic acid (37.3 g.) 90%; M.P. 210 C. Several recrystallisations from nitrobenzene gave a purified product with M.P. 218 C.-220 C. Infra-red confirmed the presence of a fluorinated aromatic nucleus and carboxyl groups (found: C, 40.3%; H, 0.9%; F, 31.0%. C H F O requires C, 40.3%; H, 0.8%; F, 31.9%).

References Cited by the Enaminer Gething et al.: Nature, volume 183 (1959), pages 588- 89.

Haszeldine et al.: J. Chem. Soc., London (1950), pages 3617 23.

Le Fave: J. Amer. Chem. Soc., volume 71 (1949), page 4148.

McBee et al.: J. Amer. Chem. Soc., volume 73 (1951), pages 1366-67.

NICHOLAS S. RIZZO, Primary Examiner.

HENRY R. JILES, Assistant Examiner. 

5. TETRAFLUOROPHTHALIC ANHYDRIDE, O-C6F4-(CO)2O. 